akg m-s stereo recording techniqueslcweb2.loc.gov/master/mbrs/recording_preservation/manuals/akg...
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M-S STEREO RECORDING
TECHNIQUES
M-S STEREO RECOR[]ING TECHNIQUES Placement of microphones for stereophonic recording is an art whose
object is the recreation of the natural tonal balance and spatial distri
bution of the original sound source and its acoustic environment. There
are two broad systems of stereophonic recording techniques used to attain
these objectives. The time intensity system uses spaced microphones in
which the stereophonic effect is due primarily to a difference in both the
arrival time and magnitude of the sound reaching each microphone. The in
tensity difference system uses a pair of microphones mounted coincidentally
i.e. on the same axis so that differences in the time of arrival at each
microphone are negligible and the stereophonic effect depends solely on
interchannel amplitude differences. (fig. 1 . )
An acoustic source picked up by a pair of directional coincident micro
phones can be recorded as mid-side (M-S) or left-right (X-Y) signals (fig. 2).
In the M-S system one of the microphones designated as a mid microphone
has a cardioid pattern which is oriented toward the sound source. The other
microphone (in the AKG C-24, the top unit) has a cosine or figure eight pattern
facing sideways with the null of the figure eight pattern coinciding with the
main axis of the cardioid. Thus the cardioid microphone picks up the sum of
all the signals present and the figure eight microphone picks up only the
left and right information. The outputs of the two microphones are then
sent to a sum and difference matrix. (fig. 3) The output of the matrix is a
normal left and right stereo signal. The output from the sum microphone
yields a compatible monophonic signal without the phase cancellations present
when spaced microphones are mixed. By attenuating either the mid or side
signal before they are matrixed the width of the sound source can be in
creased or decreased respectively.
In the X-Y technique of intensity stereo recording, a pair of identical
directional microphones are inclined to the left and right of the sound
source. The principle axis of each microphone is usually set up at an
angle of 45 to 60 degrees to the direction of the sound source. The polar
patterns of the two microphones can be set at cardioid, figure eight, or
in between depending upon the room acoustics and the properties of the sound
1
source. The scale of width of a left - right pair can be varied by con
verting the output to M-S through matrixing, attenuating the M or S signal,
then converting the M-S signal back to a left - right output with another
matrix. In principle, both techniques of M-S and X-Y recording are equivalent
and each signal can be transformed to the other by means of matrixing (sums and
differences).
Figure 4a shows the polar diagrams for pairs of coincident microphones
with different directional characteristics and figure 4b shows their M-S
equivalents after matrixing. The polar patterns from top to bottom are
figure of eight, hypercardioid, cardioid, all at 90 degrees, and cardioid
at 180 degrees (back to back). Except in the case of the cardioids at 180 de
grees, the M signal is relatively "dry" and the S signal contains most of the
reverbera tion.
To show the a ctual results obtained from matri xing the outputs of
microphones with normally used directional characteristics, a mathematical
analysis is required.
The general equation for the output of a directional microphone from
a sound at an angle y expressed in polar coordinates is
R (Y) A+Bcos (y-ao ) (1)
0 is the angle of maximum sensitivity of the microphone.0
As is well known the following patterns are described Hhen the equation is
given the respective parameters
A 0 figure eight characteristic
B 0 omnidirectional
BfA 1 true cardioid
BfA 1. 94 superca rdioid
BfA 3 hyperca rd ioid
The directional characteristic can thus be seen as the result of mixing in
various proportions, the output of an omnidirectional microphone A and of
a fi gure eight microphone B.
2
Bringing equation (1) to standard form with BIA = a and RIA = rand
setting the output of the omnidirectional microphone to 1 (A = 1) we have
equation (2)
r(y) = 1 + a cos (y-ao ) (2)
this equation can be transformed to
r( y) 1 + x cos y + y sin y (3)
+ y2 Qo = arctg
Equation 3 shows that any general cardioid pattern can be built up from the
output of an omnidirectional microphone (r = 1) and of two f~gure eight micro
phones at rightangles to each other in the main axes (r = x cos y, r=y sin y)
X-y stereophony
The equation for two crossed cardioids offset by 90 degrees to each
other is from eq. 2.
r (y)x, y = 1 + cos (y - (9D:!:~) a=l, tga = 1
transformed into the form of eq 3 where x=y= ~/2, the output of the
microphone is
r(y) =l+Vz/2 siny±V2/2cosyxy
when transformed by a matrix, the sum or M signal in standard form is
r (y) M = 1 + -..[2/2 sin y (A=2)
the difference or S signal in standard form is
r (y) S = Yz/2 cosy (A=2)
The directional characteristic of the M microphone in an equivalent MS
system corresponds to a cardioid with a predominant omnidirectional share
and a front to back ratio of 15 dB. In the case of the AKG C24 stereo
microphone this corresponds to the position between omnidirectional and
cardioid. The S signal would be that of a figure eight microphone with an
output in the ratio of 1 + 1V2 ' that is, 7.6 dB less than the level of
the M signal.
An MS microphone pair adjusted to these characteristics and levels will
provide left-right signals after matrixing equal to that delivered by two
3
XY microphones set at + 45 degrees and with true cardioid directional
characteristics.
M-S Stereophony
In normal use a true cardioid characteristic provides the M signal and a
90 degree offset figure-of-eight pattern of identical sensitivity provides
the S signal. The equation for these two characteristics according to (2)
is
r (y) M, S = 1 + siny± 2 cosy
with x = 2, Y 1530 24')
As a result of the formation of sums and differences (matrixing) we
obtain the equation in standard form of
r (y) XY = 1 + Vs cos (y - (900 ± 630 24'))
which represents two cardioid shaped directional characteristics each
at an angle of approximately 63 degrees to the main axis and of an approximate
supercardioid shape with a ratio of omnidirectional to figure eight
proportions of 1:V5 (BfA 2.24) this is very close to the supercardioid
pattern on the S24 which has the proportions BfA = 1.94.
Thus \.;re see that the matrixed output of an MS dual coincidence microphone
\.;ril1 provide the same left right information as a pair of XY microphones
set at an angle of 60 degrees to the main axis and having a super
cardioid directional characteristic. On the AKG C24 the two capsules
would be turned ±60 degrees to the axis of symmetry and the pattern selector
switch set at the position between cardioid and figure eight.
A change in directional characteristics makes it possible to adjust the
recording to any given room characteristics. Broadening the polar
characteristics from crossed figure-eights to crossed cardioids narrows
the apparent width of the sound image. At the same time it reduces the
reverberation picked up from the rear. This effect is the opposite of
that obtained by variation of the microphone distance since, in the latter
case, moving further back narrows the source but increases the reverberation.
Attenuating the M or S signal before matrixing expands or contracts the
width of the image respectively with the same effects upon the reverberation
as changing the directional characteristic.
4
Stereo requirements place a close tolerance on microphone character
istics with regard to amplitude and phase matching as well as the accuracy
with which the polar response can be maintained independent of frequency.
Further in using coincidence microphones, too great a distance between the
two units results in time differences which lead to cancellation of
certain frequencies in the compatible mono signal and time differ
ences for sound sources above or below a point halfway between the
two microphones; so that for example an actor's voice and his foot
steps appear to corne from different sideways points.
The C24 microphone was specifically designed for intensity stereo re
cording techniques. The coincident capsules are mounted with a spacing
of 3.8 cm. (l~") making any differences in time between the two outputs
negligible. This is crucial in maintaining separation between left and
right outputs at high frequencies using the M-S techni~ue. (Fig. 5)
Nine different directional patterns can be selected for each microphone
by the pattern selector S24. These patterns are identical as to their
phase relationships and sensitivity and maintain their polar character
istics independent of frequency. The patterns are omni, cardioid, figure
eight and six intermediary positions. The switch on the right side
of the pattern selector controls the rotatable (S) microphone system.
(Fig. 6)
The stand connector is designed to facilitate rapid and accurate change
over from MS to XY stereo recording technique. A window indicates the
symbol MS(2). When the stand connector(l) is rotated 45 degrees counter
clockwise the symbol XY indicates the correct XY position. The upper
microphone system can be rotated 1800 to provide any offset angle
des i red (Fig. 7)
See pages 9, 10, 11, 12 and 13 for technical description of AKG C-24.
5
Fig. 1
Fig. 2
GRN-WH )}
FROM
SHIELD
C24
)"
'~ \
PATTERN XLR-3-31 SELECTOR J NF-24 WH )...-.!.
SHIELD )'
~ ~ BLU " r RED.? IBLU-WH
RED-WH
GRN >-. ~ BLK
I"--=-G=RN;";"'--O-R--'
~ T PAD-GOO n : ~,--BL_K_-W_H-+-_t---1f--~-7>>-3___
MID TRIAD A-G7-J: : ~-+___~)y>-2______
~ _____________ _ ______ J
GRN 1.J1 BLU
CABLE ~~--7"·~3~~-~"~~~Y~-~R-ED--.'
,( IBLU-WH GRN-OR REO-WH )' RED-WH tS --~--7"·~2--~----~~--~~~:tBLK-WH
SIDE I I .L
-7>>-1_
\ XLR-3-32
'\)>--)-,. 3
)~>-2-----
TO CHASSIS
MS MATRIX CIRCUIT FOR AKG C24 STEREO MICROPHONE
Fig. 3
6
R
OUTPUT TO MIC PREAMP
L
.... --
\
\,~ \ '\
.....
---.......8',
w x '\
\ \ , -,I , ... ...I
/ " '\,I I
, I I \ I
I I/ I
S'I I
I \ II I , II I
\ I .-'" \ ,
..... ,
..... _---I
z y
w x
......... , 8\
\ \ I"
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w x
" '\ "".,,--....... "--S... ,8' / '\ / \\ , I I aI
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40 Fig. 4 4b
7
Fig. 5
Fig. 7Fig. 6
8
TECHNICAL DATA
IMPEDANCE, VOLTAGES
Electrical Impedance at 1000 Hz: 200 ohms ± 15%, balanced, ground-free, convertible to 50 ohms ± 15%
Impedance Response: extremely flat over the entire frequency range
Min. Actual Load Impedance: ~ 500 ohms ( ~ 150 ohms)
Weighted Noise Level: 2.5 [lveff (Filter CCIF 1954 DIN 45405)
Unweighted Noise Level: 8.0 [lveff
Equivalent Noise Level: < 22 db (Filter CCIF 1954 DIN 45405)
Sensitivity to at 50 Hz 0.06 v/vs/m2 = 0.3 [lv/50 m Gauss Magnetic Stray Field: at 100 Hz: 0.5 v/ vs/ m2 = 2.5 [lv/50 m Gauss
Maximum Sound Pressure Level: at a harmonic distortion of 0,5%: 150 [lbar (117.5 db SPL)
Tube: GE 6072
Plate Voltage: 120 v
Plate Current: 0.9 ma approx.
Filament Voltage : 12.6 v D.C.
Filament Current : 175 ma approx.
Power Supply: 220 v ± 10%, convertible to 110 v ± 10%, 50 - 60 Hz
IMPEDANCE RESPONSE
250
200
150
HlO
20 50 100 200 500 1000 2000 5000 10.000 20.000 Hz
9
TEe H N I CAL D AT A FREQUENCY RESPONSE CURVES
Guaranteed frequency response curve
db
40
30
20
10
o 20 50 100
. --~-
200 500
=---~ f-- H86 :...
1000 2000 5000 10.000
b
20.000
Typical frequency response curve (1 m di stance from the source of sound) Q 00
40
30
20
10
o 20 100 200 500 1000 2000 5000 10.000 20.000
Typical frequency response curve (1 m distance from the source of sound) Q 900
40
30
20
10
o 20 50 100 200 500 1000 2000 5000 10.000 20.000
Typical frequency response curve (1 m distance from the source of sound) Q 1800
50
40
f-
30
20
10
o ~
20 50 100 200 500 1000 2000 5000 10.000 20.000
10
Typical frequency response curve (1 m distance from the source of sound) 0 0°, 90°,180°
30
20
10
o 20 50 100 200 500 1000 2000 5000 10.000 20.000
Typical frequency response curve (1 m distance from the source of sound) 800
30
20
10
o 20 50 100 200 500 1000 2000 5000 10.000 20.000
Typical frequency response curve (1 m distance f rom the source of sound) 8 90°
30 ~ 1--- t-I-
20
t--10
1--""1o
20 50
- t- - r
-100 'V" zroJ 500 1000 2000 5000 10.000 20.000
Typical frequency response curve (1 m distance from the source of sound) 8 1800
30
20
10
o 20 100 200 soo 1000 2000 5000 10.000 20.000
11
TECHNICAL DATA SENSITIVITY, DIRECTIONAL CHARACTERISTICS
Type: Pressure gradient receiver with low Frequency circuit. For stereo recording (2 systems situated one above another)
Frequency Range: 30 . . . 20000 Hz
Sensitivity at 1000 Hz 1 mV/!J.bar (- 60 dbv) re o 1 v/dyne/cm 2
Microphone Rating: GM = -132 db; -41 db re o 1 mw/10 dyne/cm2
Directional Characteristics: Cardioid, Omnidirectional , Figure-oF-eight and 6 intermediate positions
Cancellation at 1000 Hz: 900 Figure-oF-eight and 1800 Cardioid ~ 25 db
Polar diagrams (photographs), taken From 1 m distance From the source of sound. Directional characteristic: Position "Cardioid"
Polar diagrams (photographs), taken From 1 m distance From the source of sound. Directional characteristic: Position between "Cardioid" and "Omnidirectional"
12
Polar diagrams (photographs). taken from 1 m distance from the source of sound. Directional characteristic: Position "Omnidirectional"
1000 Hz
Polar diagrams (photographs), taken from 1 m distance from the source of sound.
Directional characteristic Position between "Cardioid" and " Figure-of-eight"
1000 Hz
16000 Hz
Polar diagrams (photographs). taken from 1 m distance from the source of sound. Directional characteristic Position "Figure-oF-eight"
1000 Hz
16000 Hz
13
Recording in the MS or XY mode is also possible with two closely mounted AKG C-451 E F.E .T. condenser microphones. Above illustration includes 1 C-451E, with 3-pattern capsule CK-6 set at figure eight (S) and 1 C-451E with ca rd ioid capsule CK-1 (M) . Special accessories include A-51 swivel joint to tilt capsule in proper position and H-10 stereo bar stand adapter.
Patterns described on page 7 can be obtained with C-451E by using different combinations of capsules (Details are available by requesting AKG's special " C-451 E CMS" booklet) .
MICROPHONES • HEADPHONES DISTRIBUTED BY ~ NORTH AME RIC AN PHILIPS CO RP OR ATI ON 100 EAST .~nd STREET. NEW YORK, N E W YOR K 10017